The two principal pulsed emitter direction finding techniques of the prior act are monopulse and sequential lobing. (CONSCAN is considered a special extension of sequential lobing, therefore, sequential lobing as used herein is intended to include CONSCAN). Basically, monopulse determines direction on a single received pulse by comparing the response of a minimum of two receiving channels, but directional accuracy improves when more than two sets of antennas and associated receiving channels are employed, four such sets being typically employed in a given installation. Since the receiving channels process the same pulse, the difference in amplitude from pulse to pulse does not affect the directional measurement. In sequential lobing successive pulses are typically measured with either a single antenna deflected at different angles and feeding into a single receiving channel or a plurality of antennas set at fixed fixed deflection angles which are sequentially switched into a single receiving channel. Through a knowledge of the antenna patterns, the angle of deflection and the amplitude of the received signal from each antenna on successive pulses, the direction of the emitter can be ascertained; however, if the transmitted pulse-to-pulse level varies, the directional accuracy of the sequential lobing technique is deteriorated.
A special form of sequential lobing has been used in an attempt to overcome pulse-to-pulse variations by sequencing through the directional antennas within the time of a single pulse. Since various portions of the same pulse are compared to determine direction, pulse-to-pulse variations are eliminated. Unfortunately, a number of disadvantages occur with this system which seriously reduce its usefulness including errors incurred by pulse amplitude variation within the pulse. Received radar pulses tend to be non-uniform in amplitude over the pulse width. Obvious variations occur during rise and fall time, but other variations also occur during the peak of the pulse. Additional disadvantages are related to the hardware required to switch at the high speed necessary to sequence through the directional antennas during a single pulse width. These include the need for high speed switches, and wide band amplifiers to handle the narrow chopped pulses. Inherent in the use of wide band amplifiers is lower sensitivity. High speed switches often produce transients which are transmitted through the remainder of the receiving system. The present invention avoids these disadvantages of this special form of sequential lobing as well as a number of disadvantages associated with standard sequential lobing and monopulse.
The most significant advantage of the monopulse system is its ability to determine direction independent of pulse-to-pulse variation. Its disadvatnage is the need for a receiving channel for each antenna used. The present invention maintains the advantage of independence from pulse-to-pulse amplitude variation found in monopulse systems while removing its major disadvantage; with the present invention only two channels are required even though more than two directional antennas are used to produce a high degree of direction detection accuracy.